1. Field of the Invention
This invention relates to an improved process for the drying of water-wet solids of various kinds, for solvent-extraction of indigenous solvent-soluble compounds from said solids, and for recovering solids, water, and indigenous solvent-soluble compounds for further use. More particularly, it deals with improvements to a continuous process for drying and solvent-extraction of water-wet solids wherein the input material is mixed with water-immiscible solvents of various kinds to obtain a mixture that remains fluid and pumpable even after virtually all of the water has been removed (at least 85% of the water present in the feedstock), and to extract indigenous solvent-soluble compounds from the input material.
2. Discussion of the Prior Art
Drying and solvent-extraction of water-wet solids is the object of large and varied industries. Examples of water-wet solids requiring such treatment include, but are not limited to:
(1) Municipal and industrial sewage sludges, such as raw primary sludges, waste activated sludges, anaerobically digested sludges, and bio-sludges; PA1 (2) Animal wastes, such as pig manures, wool-scouring wastes, chicken manures, and cow manures; PA1 (3) Contaminated soils, such as soils contaminated with crude oils, fuel oils, polychlorinated biphenyls, polynuclear aromatics, coal tars, and oil drilling muds; PA1 (4) Refinery sludges, such as API separator sludges, dissolved air flotation floats, and slop oil emulsion solids; PA1 (5) Ink and dye sludges; PA1 (6) Alum sludges; PA1 (7) Wood pulp mill activated sludges and black liquors; PA1 (8) Pharmaceutical plant wastes; PA1 (9) Brewery sludges; PA1 (10) Dairy and food products and wastes, such as milk whey by-products, coffee wastes, and chocolate wastes; PA1 (11) Peats, lignites, and brown coals; and PA1 (12) Meat rendering wastes.
Drying and solvent-extraction of water-wet solids present many processing problems relative to the efficiency and reliability of production. Various typical processes for dehydrating water-wet solids using solvent extraction technologies are disclosed in U.S. Pat. Nos. Re 26,317; Re 26,352; 3,323,575; 3,716,458; 3,855,079; 3,950,230; 4,013,516; 4,270,974; 4,418,458; 4,336,101; 4,702,798; 5,076,895; 5,256,251; and 5,518,621.
In general, the processes and apparatus described in the aforementioned patents involve slurrying water-wet solids, such as one or a combination of the types listed above, with a water-immiscible solvent to obtain a mixture which remains fluid and pumpable even after virtually all of the water has been removed. The properties of the solvent can be varied over a wide range to achieve the desired characteristics. The solvent should be immiscible in water and should have an atmospheric boiling point of 300.degree. F. or higher to prevent excessive evaporation of the solvent during the evaporation of water from the solvent. The viscosity of the solvent should be low enough, typically less than 500 cp, so that the slurry is pumpable at the flowing temperatures. Extraction of compounds from the input water-wet solids can be enhanced by changing the chemical composition of the solvent to increase the solubility of the compounds in the solvent. The chemical composition of the solvent can also be adjusted to improve the dispersibility of the water-wet solids in the solvent. Isopar "L" and Ashland 140 Solvent 66 are the trade names for solvents which meet the above criteria and have been used in these processes. Iso-octanol is an example of another solvent which has been used in these processes.
The resulting mixture of solvent and water-wet solids is passed through a sequence of drying steps in which the mixture is dried by heat evaporation. Economies of energy consumption are realized by utilizing the evolved vapor from each evaporation step except for one, typically the first step to supply a substantial portion of the heat requirements of another evaporation step. The evaporation steps generate a slurry of dried or partially-dried solids in solvent which is withdrawn and fed to a centrifuge (or other apparatus for separating liquids from solids) to separate a substantial portion of the solvent from the solids. The solids leaving the centrifuge are sometimes processed further by heating them in a "desolventizer", referred to as a cake deoiler and an example of which is disclosed in U.S. Pat. No. 4,270,974. In the desolventizer, blowing steam, purge gas, and/or vacuum are used to recover most of the remaining solvent from the solids. In many cases, the centrifuge centrate is fed to a distillation system where the indigenous solvent-soluble compounds extracted from the solids are separated from the solvent and recovered for final disposition.
An important consideration in the design of a multi-effect evaporation drying facility is to select optimum operating conditions for the evaporation steps. Water evaporation rates are directly proportional to the temperature differentials between the condensing steam vapor on one side of the evaporator system heat exchangers and the material being dried on the other side of the heat exchangers. To create temperature differentials, the condensing temperature of the steam leaving some of the effects is reduced by operating the stages under vacuum; and to maximize the temperature differentials available, the lowest practical pressures are used. Thus, in the prior art, a common example would be to have three evaporators, with the first evaporator operating at about 1.5 psia, the third evaporator operating at atmospheric pressure (14.7 psia) and the second evaporator operating at an intermediate pressure. In a simplified example, the operating temperature (and the condensing temperature of the steam generated in that stage) would be about 116.degree. F. in the first stage whereas the steam leaving the third evaporator would have a condensing temperature of about 212.degree. F. Thus, the temperature differential to provide evaporation in the heat exchangers for the first and second evaporators would be about 96.degree. F.
For example:
______________________________________ Operating Operating "Steam" Condensing Pressure Temperature Temperature ______________________________________ 1st Stage 1.5 psia 116.degree. F. 116.degree. F. 2nd Stage 5.2 164 164 3rd Stage 14.7 212 212 Temperature Differentials: (3rd Stage Steam - 2nd Stage Operating) = 48.degree. F. (2nd Stage Steam - 1st Stage Operating) = 48 Total 96.degree. F. ______________________________________
However, all of this differential is not available to create heat transfer in the evaporator heat exchangers due to a phenomenon known as a boiling point rise. In prior practice, it had been found that the measured boiling point temperature of water when in contact with most types of solids is greater than the boiling point temperature of water at the same pressure when the solids are not present. This temperature difference is called a boiling point rise (BPR).
In practice, the BPR's for all stages in an evaporator system must be subtracted from the total temperature differential in order to determine the net amount of temperature differential available for heat transfer. Thus, in the above example, if the BPR's in the first and second evaporators were 10.degree. F. each, the temperature differential available to provide evaporation in the first and second stage heat exchangers would total 96.degree. F. minus 20.degree. F. or 76.degree. F.
To illustrate:
______________________________________ Operating Operating "Steam" Condensing Pressure Temperature BPR Temperature ______________________________________ 1st Stage 1.5 psia 126.degree. F. 10.degree. F. 116.degree. F. 2nd Stage 5.2 174 10 164 3rd Stage 14.7 262 50 212 Temperature Differentials: (3rd Stage Steam - 2nd Stage Operating) = 38.degree. F. (2nd Stage Steam - 1st Stage Operating) = 38 Total 76.degree. F. ______________________________________
In the prior art, it was believed that as the ratio of solids to water as increased and the BPR increased also; however,the BPR was strictly a function of the solids/water ratio and that for a particular solid, the BPR was unaffected by other variables, such as pressure or temperature. Thus, when practicing the prior art in designing or operating a multi-stage evaporator system, all that was required to be known was the solids/water ratio of the material being dried in a particular stage; this quantity then determined the BPR and from it the required design basis or operating conditions could be set. Furthermore, if it were desired to design or operate the evaporator system at a different pressure or temperature, only the solids/water ratio would be needed to predict the performance at the the new conditions. If the solids/water ratio was the same as that used previously, no additional data would be needed to predict the new performance.
However, for the present invention, it has been discovered that the BPR is actually a function of solids/water ratio and evaporator operating pressure and temperature. It was found through experimentation that different curves of BPR point rise versus solids/water ratio are obtained at different temperatures. In general, the higher the operating pressure and temperature, the lower the BPR for a given solids/water ratio.
Thus, if one were to contemplate the effect of increasing the operating pressure of the atmospheric pressure evaporator to significantly higher pressures, the associated higher operating temperature provides some unexpected benefits: Not only does the total temperature differential between the lowest and highest pressure evaporators increase, but the BPR unexpectedly declines, resulting in a larger than expected temperature differential available for heat transfer in the evaporator heat exchangers.